1. Technical Field
The present invention relates to piezoelectric materials that are used for piezoelectric elements.
2. Related Art
Piezoelectric elements possess a characteristic in which crystals are charged when they are deformed, or they are deformed when they are placed in an electric filed. As such, they are commonly used for liquid jet apparatuses such as ink jet printers.
Piezoelectric thin films such as PZT (lead titanate zirconate: Pb(ZrxTi1-x)O3) films have been widely used for such piezoelectric elements.
Unfortunately, PZT films contain lead (Pb), and therefore could pose problems for the safety of operators and the environment. Accordingly, research and development regarding lead free piezoelectric materials are being extensively conducted. For example, Japanese Laid-open Patent Applications JP-A-2004-6722, JP-A-2007-266346 and JP-A-2007-287745 describe lead free piezoelectric materials.
Among the aforementioned piezoelectric elements, thin film piezoelectric elements in which a thin film piezoelectric layer is sandwiched between electrodes have been studied and developed in recent years. Thin film piezoelectric elements are generally formed by a process in which a piezoelectric layer having a film thickness of several μm or less is formed on an electrode that is formed on a substrate, and an upper electrode is formed on the piezoelectric layer. This process is conducted through micro-processing that is similar to a semiconductor process. Therefore, thin film piezoelectric elements have an advantage in that piezoelectric elements can be formed at a higher density than ordinary piezoelectric elements.
Although PZT is often used as the piezoelectric material for thin film piezoelectric elements, the use of lead free piezoelectric materials is now also being considered.
In fact, the inventors of the present invention have been conducting research and development on piezoelectric materials, and examining the application of lead free piezoelectric materials and the improvement of lead free piezoelectric material characteristics.
BaTiO3 and a variety of materials containing Bi may be representative examples of lead free piezoelectric materials. BaTiO3 has a large piezoelectric constant but has a low Curie temperature of about 120° C., and thus has a problem with temperature stability. Many of the materials containing Bi have a high Curie temperature, but many of them have certain problems, such as a small dielectric constant and a small spontaneous polarization, as well as problems in temperature stability due to the phase transfer temperature being lower than the Curie temperature. Thus, although lead free piezoelectric materials are being widely studied and developed as described above, in reality they have both advantages and disadvantages.
Other problems that may be encountered when using lead free piezoelectric materials for thin film piezoelectric elements include a compositional shift due to the fallout of alkali metal elements in materials containing such alkali metal elements, and diffusion into lower electrode layers due to the high temperature crystallization necessary for BaTiO3. In either case, it is difficult to use these materials for thin film piezoelectric elements.
BiFeO3 has a high Curie temperature and a large spontaneous polarization, and also has many other advantages suitable for thin film formation. For example, BiFeO3 does not have elements that would likely cause compositional shift, such as alkali metal elements, and has a lower crystallization temperature, such that BiFeO3 is attracting attention as a material for ferroelectric memories. Thus, attempts to use BiFeO3 as piezoelectric material have also been made in recent years. However, BiFeO3 has a small dielectric constant, and thus cannot be expected to achieve a high piezoelectric constant as a stand alone material.
In view of the foregoing, there has been a study to increase the piezoelectric constant by mixing crystals of BiFeO3 and BaTiO3. However, when BaTiO3 is mixed too much, BaTiO3 becomes dominant and as a result the high Curie temperature and large spontaneous polarization of BiFeO3 cannot be best used. Also, if piezoelectric elements are manufactured independently with either of the materials, BiFeO3 would likely result in a poor dielectric property, and BaTiO3 would deteriorate the piezoelectric characteristics since it is difficult to make its composition uniform, and thus would likely form layers with a lower dielectric constant. Therefore it is necessary to devise a solution for controlling the shortcomings of the respective materials, and to select appropriate mixing ratios.